1. Field of the Invention
The present invention relates to an interconnector for connecting electronic device elements electrically in serial or parallel direction and to an electronic device element using the same. More specifically, the present invention relates to an improvement in a planar type interconnector used for solar cells for artificial satellite or diodes, and to electronic device elements such as solar cells and diodes using the same.
2. Description of the Background Art
Interconnectors which are metal strips for electrically connecting a plurality of electronic device elements in serial or parallel direction are broadly classified into three dimensional ones and planar ones dependent on whether a stress relief which is a portion for absorbing displacement generated between elements connected with each other has three dimensional shape or a planar shape. A prior art example of the planar type interconnector, to which the present invention is directed, will be described with reference to solar cells mounted on an artificial satellite, by way of example.
FIG. 13 shows a planar type interconnector for solar cells mounted on an artificial satellite. An interconnector 1 shown in FIG. 13 is provided with a front surface electrode connecting portion 2 which is connected to a front surface electrode of a solar cell, and a rear surface electrode connecting portion 3 which is connected to a rear surface electrode of another solar cell which is neighboring the former solar cell in the serial direction. At a region between front surface electrode connecting portion 2 and rear surface electrode connecting portion 3, a stress relief portion 4 for absorbing displacement generated between front surface electrode connecting portion 2 and rear surface electrode connecting portion 3 is provided.
Stress relief portion 4 has two approximately parallel slits 5 and 6 for absorbing displacement. Slits 5 and 6 have closed ends 5a and 6a with approximately circular notches the diameter of which is larger than the width of the slit, and open ends 5b and 6b, respectively.
The width of stress relief portion 4 in the parallel direction (left and right direction of FIG. 13), that is, the distance W2 between left and right ends is the same as the width W1 of front surface electrode connecting portion 2 in the parallel direction as well as the width W3 of rear surface electrode connecting portion 3 in the parallel direction. If the largest value of the width in the parallel direction in the interconnector 1 as a whole is represented as W4, the widths W1, W2, W3 and W4 are all the same in interconnector 1 of this prior art example.
The conventional interconnector for solar cells such as described above is disclosed, for example, in Japanese Patent Laying-Open Nos. 1-198082 and 4-284677.
A solar paddle for artificial satellite generally used in the prior art employing the above-described conventional interconnector 1 will be described with reference to FIGS. 14A and 14B. In the solar paddle shown in FIGS. 14A and 14B, rear surface electrode connecting portion 3 of interconnector 1 is connected to the rear surface electrode of solar cell 7 and adhere on a substrate 9 by means of an adhesive 8. Front surface electrode connecting portion 2 is connected to the front surface electrode of solar cell 10 which is next to solar cell 7 in the serial direction, that is, the longitudinal direction of FIG. 14A, and solar cell 10 is also adhered on substrate 9 by adhesive 8.
When solar paddle is used in an environment such as space in which temperature cycle ranges widely from about -180.degree. C. to +100.degree. C., for example, the distance between rear surface electrode connecting portion 3 and front surface electrode connecting portion 2 of interconnector 1 connecting solar cells 8 and 10 changes, because of the difference in thermal characteristics (for example coefficient of thermal expansion) of solar cells 7 and 10, substrate 9, interconnector 1 and adhesive 8, respectively. The change of the distance is absorbed at stress relief portion 4 by means of the change of the width of slits 5, 6 and then the stress is relaxed.
FIG. 15 is a plan view showing a plurality of solar cells connected not only in serial direction but in parallel direction. In this case, in addition to interconnectors 1 for serial connection, interconnectors 12 for connecting rear surface electrodes of solar cells in parallel directions are used.
FIG. 16 is a plan view of another example of solar cells connected both in serial and parallel directions, in the similar manner as shown in FIG. 16. As for the manner of connection in parallel direction of the solar cells in the example of FIG. 16, connection in both the serial and parallel directions is effected by one interconnector 13. Since interconnector 13 has a connecting portion 13a for parallel connection extending in the parallel direction, the width thereof as a whole is larger as compared with the interconnector 1 used only for serial connection.
The conventional interconnectors 1, 12 and 13 described above have the following disadvantages.
In the conventional interconnector 1 for serial connection shown in FIG. 13, the width W1 of front surface electrode connecting portion 2 is the same as the width W4 of the whole interconnector 1 as well as the width W2 of stress relief portion 4, and therefore, the width of the front surface electrode of solar cell connected to interconnector 1 must have a width at least wider than the width W1 of front surface electrode connecting portion 2, that is, wider than the width W4 of the whole interconnector 1. Accordingly, the front surface electrode of the solar cell occupies large area of the light receiving surface of the solar cell, reducing effective light receiving area with respect to the entire planar area of the solar cell, that is, reducing the ratio of area having the function of effectively converting the received light to power. This prevents improvement of power generating efficiency of the solar cells.
When solar cells connected in serial direction by using interconnector 1 are to be connected in parallel as shown in FIG. 15, separate interconnectors 12 for parallel connection are required. This increases the number of parts constituting the solar paddle, increasing the cost for assembly.
Further, when solar cells are to be connected both in the serial and parallel directions by means of one interconnector 13 as shown in FIG. 16, the width of interconnector 13 as a whole is further increased since it has a rear surface electrode connecting portion 13a for parallel connection and a stress relief portion 13b for absorbing displacement generated in the parallel direction, and therefore handling of the connector is troublesome. Further, the interconnector 13 is not suitable for connecting solar cells only in the serial direction, as the connecting portion for parallel connection is a hindrance.
A conventional thin type diode element with interconnectors for space use, connected by interconnectors having approximately the same shape as the above described conventional interconnector 1 will be described. FIG. 17A is a plan view and FIG. 17B is a cross section thereof. For convenience, a reflecting plate 39 and an insulating film 35 of FIG. 17B are omitted in FIG. 17A.
Referring to FIGS. 17A and 17B, a semiconductor substrate 31 is used as a substrate for a diode 42. Semiconductor substrate 31 is of a p or n type silicon single crystal having the size of about 1 to about 10 cm square and thickness of about 0.1 to 0.5 mm. By forming a diffusion layer 32 having opposite conductivity type to the impurities of the substrate, a pn junction 33 is formed, which function as a diode. The surface of the substrate is covered by an oxide film 34, which is further covered by an insulating film 35 thereon. An opening 34a is provided in oxide film 34 on which opening 34a metal is deposited to form an electrode 36. By depositing metal on the rear surface of the substrate, another electrode 37 is formed. One end of the interconnector 1 is connected to electrode 36 by welding.
On the surface of the conventional diode element, a reflecting plate 39 (an aluminum plate, a mirror or the like) is adhered by an adhesive 40 in order to improve radiation in the space, and to prevent entrance of sunlight to the semiconductor substrate to reduce leak current in reverse direction of the diode caused by photoelectric current. Semiconductor substrate 31 and interconnector 1 are insulated from each other by oxide film 34, insulating film 35 and adhesive 40. A mesa portion 41 may be formed by mesa etching on the side surface portion so as to enlarge distance between the semiconductor substrate 31 and interconnector 1. Insulation between interconnector 1 and semiconductor substrate 31 is indispensable and when this insulation fails, the diode loses its function.
The shape of interconnector 1, referring to FIG. 17A, is different from that of the interconnector 1 shown in FIG. 13 only in that front surface electrode connecting portion 2 is longer. At inner ends of slits 5 and 6, approximately circular notches 5a and 6a are provided to prevent concentration of stress. A connecting piece 52 at one end of interconnector 1 is connected to electrode 36 of diode 42. Connecting portion 3 at the other end of interconnector 1 is connected to an electrode of neighboring diode or solar cell.
As for interconnector 1 for connecting diodes of the prior art described above, since the width of interconnector 1 at both ends is uniformly wide, the area of the electrode to be connected thereto must also be wide.
Referring to FIG. 17B, semiconductor substrate 31 and interconnector 1 are insulated from each other by oxide film 34, insulating film 35 and adhesive 40. When moisture or foreign matter such as very small dust enters between semiconductor substrate 31 and interconnector 1 at the side surface portion, for example, the pn junction of diode 42 may possibly be short circuited. Since electrode 36 and interconnector 1 are insulated from reflecting plate 39 only by adhesive 40, there is also a possibility of short circuit caused by foreign matter, if any, between semiconductor substrate 31 and insulating plate 39, if the insulation is not sufficient.
When the thin plate type diode is manufactured, since it is necessary to make the mesa portion formed by mesa etching to be wider than the width of the interconnector, it is difficult to provide semiconductor substrate 31 with sufficient strength, resulting in the problem of breakage during the process of manufacturing.